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Practical Problems 

in Mechanical Drawing 

and Blue-Print 

Reading 



STURTEVANT 

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PRACTICAL PROBLEMS 

In Mechanical Drawing and 
Blue-Print Reading 

By WrW. Sturtevant 
Instructor of Mechanical Drawing , 

at South High School 
Minneapolis, Minn. 

Formerly instructor of Mechanical Drawing at the Jackson Grade 

School, the Bremer Junior High School and Evening and 

Part-Time Trade Classes in Minneapolis. 

Copyright, 1921, by W. W. Sturtevant 



Published by 

PRACTICAL PROBLEMS CO. 

3332 Forty-fourth Ave. S. 
Minneapolis, Minn. 



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TO THE TEACHER 



Much criticism has been directed at the "copy" method of teaching Mechanical Drawing 
and undoubtedly some pupils have copied drawings which they did not clearly understand. 

The attempt, however, to make a problem of every drawing has been' responsible for much 
poor teaching. Many boys have gained the impression that a third view was always necessary, 
when as a matter of fact, two views of an object gave all of the information necessary and the 
drawing of the third view was a waste of time. Similarly if dotted lines on a view <|ive the 
pupil a clear idea of an object, the drawing of a section is poor practice. 

It is suggested that the test drawing, the side view or section or sketch be made on a sep- 
arate sheet in pencil, thus saving time and avoiding the possibility of the pupils receiving the 
impression that it must be made to complete the drawing. 

Acknowledgement is here made of the many helpful suggestions of my supervisors and co- 
workers in the Minneapolis Public Schools in the selection of these plates. 

W. W. Sturtevant. 



DEC 16 1921 



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14 



TANGENTS 

In the development of good technic in Me- 
chanical Drawing there is probably no point 
that presents greater difficulty than the gain- 
ing of a smoothness of tangent line's and arcs. 

The following plates are offered as illustrat- 
ing the several different problems that confront 
the pupil. • 

The two plates showing the eight problems 
should be drawn by the' pupil, after the usual 
explanation by the teacher. With these plates 
at hand the average boy should encounter no 
great difficulty in drawing any of the other 
plates. 

The construction lines on the first two plate's 
make clear the method of locating the center 
of the arc by drawing lines parallel to the 
straight lines and circles concentric with the 
circles. 

The point of tange'ncy is on a radial line 



drawn perpendicular to the straight line and 
on a line drawn through the center of the 
circle and the center of the small arc. 

Each boy is to draw as many of the plates as 
is necessary in order to de'velop the required 
skill. No pupil is to be asked to draw all of 
the plates, but by having at hand a number 
of plates the teacher may select one that meets 
the needs of the' pupil and is within the limits 
of his ability. 

By giving each boy his own problem much 
greater interest may be secured th°an by hav- 
ing the whole class draw the same plate. Also 
by asking boys to check the drawjngs made 
by othe'rs in the class they will learn much. 

The ability to visualize may be developed by 
asking for the side view or the top view, which 
may be drawn on the same or a different sheet. 
If the time ne'cessary to complete the third 
view is not available a free hand sketch may 
be substituted for this. 



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29 



SECTIONAL VIEWS. 

Frequently drawings are required of objects 
that because of their construction require a 
gre'at number of dotted lines to show the loca- 
tion of hidden parts. Such drawings are usual- 
ly difficult to understand. Especially is this 
true if the dotted lines intersect. 

To add clearness, in such cases sectional 
views or sections are usually used. The object 
is drawn as though the' section or part of the 
object nearest the observer were broken away 
and the interior exposed to view. 

The line on which the section, is taken is an 
imaginary plane, passing through the' object, 
called a trace. It need not be a straight line. 
Any line that will show the object to the best 
advantage may be used. This cutting plane is 
purely imaginary and any sections may be 
drawij that in the draftsman's opinion will give' 
added clearness. 

Only one view is, as a rule, shown in section 
the other views of the drawing show the object 
complete. 

On the drawing, the surfaces which are as- 
sumed to be cut are covered with light slanting 



lines, known as section lines or sometimes as 
cross-hatching. Section lines are usually drawn 
at an angle of 45° but in cases where the ap- 
pearance would be improved the angle may be 
change'd. These lines should be about one half 
of the weight of ihe standard line and they 
should be spaced a uniform distance apart. 

The spacing varies somewhat, being greater 
on large surfaces and less on small. Usually 
1/16 of an inch apart is about right. On a 
large surface 1/8 or 3/16 is better. Very large 
surfaces are frequently lined only around the 
edge and very small surfaces are filled in solid. 

On the pencil drawing the surface to be sec- 
tioned is indicated but the section lines are 
drawn on the tracing or ink drawing only. 
Lines are spaced by the eye, no attempt is 
made to lay them off equal distance by 
measure. 

Standard section lining to represent different 
materials are shown on page 33, but these are 
not in universal use. Many drafting rooms 
draw all materials with the same lining use'd 
for cast iron and designate the material by let- 
ters, abbreviating the name, placed on the sur- 
face to be cross-hatched. 



30 



As a shop note usually specifies the' material 
the tedious and expensive section lining to 
show cast steel, wrought iron, etc., is not con- 
sidered necessary. 

Some draftsmen fill in sectioned surface's 
with a soft pencil on the tracing. This shows 
clearly on the blue print and has the advantage 
of being very quickly done. 

Some exceptions to the rules governing draw- 
ing are' made on sectional views. These con- 
ventions are listed below. 

Dotted lines showing invisible lines back of 
a section may be drawn in or omitted as de- 
sired. 

Spokes are never drawn in section. The 
drawing is made as though the' cutting plane 
passed in front of the spoke. This makes clea,r 
the difference between wheels having spokes 
and those cast solid. 

Webs are not drawn in section. Webs are 
sometimes sectioned as in page 39. By omit- 
ting alternate lines the web is made easy to 
distinguish. 

Objects that are symme'trical about an axis 
are usually drawn one half in section. 

Unsymmetrical objects, as wheels with 3 or 



5 spokes, are drawn as though they were 
symmetrical. 

Adjacent surfaces of different pieces are sec- 
tioned in a different manner or with lines 
drawn at a different angle. 

The draftsman must use his judgment as to 
the number of sectional views that are neces- 
sary. Sometimes three or four sections are 
necessary. Again a revolved section of a very 
small part is all that is nee'ded. 

Often the shape of a spoke or a web or 
similar small part of an object may be showh 
by what is called a revolved se'ction. This sec- 
tion is placed on the regular view. 

The pupil should bear in mind the fact that 
a section takes more time and therefore costs 
more than the drawing of a side view, and 
should be used only when the added clearness 
gained will warrant the -extra time necessary 
in the drafting room. 

Solid cylindrical objects, like bolts or shafts, 
are not drawn in section. 

Removed sections are sections drawn in some 
convenient place on ti.e sheet, out of their na- 
tural position. 

Partial sections are used when a section of a 
small part of an object will show all that is 
nece'ssary. 



31 



CAST IRON 

STE5L SPRING 




VALVE STEM PACKING 



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53 



AUXILIARY VIEWS 

Auxiliary views are used when it is neces- 
sary to show the true length of a slanting sur- 
face that would appear fore-shortened in the 
top or side view. 

The center line of the auxiliary view is 
drawn parallel to 'the surface to be shown in 
this view. 

DIMENSIONS 

The dimensions on a drawing are of the 
utmost importance and the correct selection of 
the dimensions to be' given and the proper 
placing of these dimensions requires good judg- 
me'nt and a considerable knowledge of shop 
practice. 

Without the dimensions the drawing is of 
little or no value. It shows the shape of the 
object but the exact sizes are necessary before 
it is of value to the workman. 

To dimension a' drawing properly the drafts- 
man should be familiar with the methods and 
machines used by the mechanic who is to make 
the object he is drawing, as this knowledge 
will enable him to give the information in the 
best way and may save the mechanic much an- 
noyance. 

The following are some general rules which 
it is well to follow, but it must be borne in mind 
that there are exceptions to all rules and if 



added clearness or a saving of time will result 
one should not hesitate to violate any of them. 

Dimension lines should present a sharp con- 
trast to the lines of the drawing. They are 
usually made about one half the weight of the" 
standard line. Most draftsman use a continu- 
ous light line with a space left near the center 
for the figures. Some use a line made up of 
long dashes about I14 to 1% inches long separ- 
ated by a small space. 

Red ink is sometimes used for center lines. 
This makes a contrast on the blue print as the 
red lines print very light. If red ink is used 
the figures and the arrow heads are made in 
black ink. 

Sharp arrow heads are placed at the ends of 
/the dimension lines. Arrow heads should- be 
about 3/16 of an inch in length and 3/32 of an 
inch in width at the wide end. 

In structural work figures are sometimes 
placed above the dimension lines instead of in 
a space left for them near the center of the 
line. 

Extension lines are the same weight as the 
dimension lines or about one half the weight 
of the outline. They should not touch the 
drawing. A space of about 1/8 of an inch 
should be left between the end of the exten- 
sion line and the part of the drawing to be 



dimensioned. The extension lines should ex- 
tend about 3/16 of an inch beyond the arrow 
heads of the dimension lines. 

The dimensions that appear on a drawing 
should be those' of the object regardless of the 
scale of the drawing. 

Figures should be plain arid not too small. 
A common mistake of beginners is to confuse 
minuteness with neatness. Figures should be 
neat but not smaller than 1/8 of an inch in 
height. 

From 3/16 to 1/8 of an inch ^s good size. 
Do not use fancy figure's. Plain readable figures 
that allow of no mistake are best. 

Dimensions should be given in the fractions 
of an inch that are given on the machinists' 
scale, halves, fourths, eighths, etc., to sixty 
fourths. Other fractions should be given in 
decimals as 1.33" or 2.23". In some drafting 
rooms decimals are used entirely. 

It is customary in many drafting rooms to 
+ .003, 
give a limit of accuracy, as 4. — .0 , meaning 
that the finished object may exceed 4 inches by 
not more than 3/1000 of an inch. 

4. — .005 means that the object may be 
5/1000 smaller, but no larger. 

4. + .003 would indicate that the finished 
piece might be either 3/1000 larger or 3/1000 



smaller; but must not exceed that limit. 

Fractions should be made with the line be- 
tween the numerator and the denominator 
horizontal, not slanting. Thus J, not 1/4. The 
slant line allows the possibility of a mistake, 
as in 1 5/16 or 15/16. 

Dimensions should, if possible', be kept off 
of the drawing and placed at the side or be- 
tween views. A good drawing should stand 
out clearly and each dimension line placed on 
it adds to the confusion. 

Give the diameters of circles and the' radii 
of arcs. Diameters should be followed by the 
abbreviation, Diam., Dia., or D. ; radii, by Rad. 
or R. 

Diameters are usually given for circles be- 
cause a round object is usually turned on the 
lathe and the workman will measm'e with cali- 
pers. If the radius is given on the drawing, 
he must multiply that by two and might make 
a mistake. 

Holes are drilled with drills marked accord- 
ing to their diamete'rs, as %", 1", etc. 

Arcs are laid out with the compasses or 
dividers and the workman must use the radius. 

Never place dimensions on a drawing so that 
it is necessary for the shop man to make cal- 
culations. Do not ask him to add several di- 
mensions in orde'r to get the total. Do not 



56 



ask him to subtract in order to find the size 
of a part. The place for calculations is in the 
drafting room. Give all necessary dimensions. 

Do not duplicate dimensions. Frequently it 
is convenient to change one or two dimensions 
on a drawing and thus save' making a new 
drawing. In such cases, if dimensions appear 
in more than one place, one may be changed 
and the other overlooked. 

Figures should read at right angle's to dimen- 
sion lines. 

Figures should read from the bottom or right 
of the sheet. Never place figures so that they 
read from the top or left of the sheet. 

Dime-nsions on diagonal lines should change 
at 60°. 

Dimension from center lines and from 
finished surfaces. 

Use notes freely when by so doing you can 
save time. 

Dimensions up to two feet should be given 
in inches; over that distance' in feet and 
inches. 

Dimensions for the sheet metal shop are all 
given in inches. 

The sizes of wheels, gears, pulleys, and cylin- 
der bores; the stroke" of pistons and the length 



of wheel bases are usually given in inches. 

Keep dimensions off of sections, if possible. 

Neve^ use center lines as dime'nsion lines. 

Never place dimensions on lines of a draw- 
ing. 

When it is desired to dimension small spaces 
where figures would appear crowde'd, the 
figures may be placed outside' of the space. 

The dimensions of angles are given on a line 
drawn with the compass set at the vertex of 
the angle. 

BLUE-PRINT READING 

Blue-print reading, properly taught, should 
give the pupil three things; first the ability to 
make a free-hand working drawing, second the 
ability to make perspective sketches and last 
and most important the ability to visualize. 

This is more likely to be of value than the 
ability to make Mechanial Drawings as many 
more boys will be engaged in work, where they 
will be required to read drawings, than will go 
into the drafting room. 

It is suggested that the pupil be asked . to 
study the drawing and then, by means of a 
sketch or a clay model give exp'ression to his 
idea of the shape of the object. 

A few lessons in Cabinet and Isometric 
Drawing should precede the work in Perspec- 
tive Drawing. 



57 




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